Saving CGContextRef

Question

I have a drawing app in which I would like to create an undo method. The drawing takes place inside the TouchesMoved: method.

I am trying to create a CGContextRef and push it to the stack OR save it in a context property that can be restored later but am not having any luck. Any advice would be great. Here is what I have ...

UIImageView      *drawingSurface;
CGContextRef       undoContext;


- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
 UIGraphicsBeginImageContext(self.view.frame.size);
 CGContextRef context = UIGraphicsGetCurrentContext();
 [drawingSurface.image drawInRect:CGRectMake(0, 0, drawingSurface.image.size.width, drawingSurface.image.size.height)]; 
 UIGraphicsPushContext(context);

        // also tried but cant figure how to restore it
        undoContext = context;

 UIGraphicsEndImageContext();
}

Then I have a method triggered by my undo button ...

- (IBAction)restoreUndoImage {
 UIGraphicsBeginImageContext(self.view.frame.size);
 UIGraphicsPopContext();
 drawingSurface.image = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();
}

When I run this, I believe my drawingSurface is being assigned nil because it just erases everything in the image.

My guess is I can't use pop and push this way. But I can't seem to figure out how to just save the context and then push it back onto the drawingSurface. Hmmmm. Any help would be ... well ... helpfull. Thanks in advance -

And, just for reference, here is what I am doing to draw to the screen, which is working great. This is inside my TouchesMoved:

 UIGraphicsBeginImageContext(self.view.frame.size);
 CGContextRef context = UIGraphicsGetCurrentContext();
 [drawingSurface.image drawInRect:CGRectMake(0, 0, drawingSurface.image.size.width, drawingSurface.image.size.height)]; 

 CGContextSetLineCap(context, kCGLineCapRound); //kCGLineCapSquare, kCGLineCapButt, kCGLineCapRound
 CGContextSetLineWidth(context, self.brush.size); // for size

 CGContextSetStrokeColorWithColor (context,[currentColor CGColor]);

 CGContextBeginPath(context);
 CGContextMoveToPoint(context, lastPoint.x, lastPoint.y);
 CGContextAddLineToPoint(context, currentPoint.x, currentPoint.y);
 CGContextStrokePath(context);
 drawingSurface.image = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();